A cutting insert for groove machining is often used for machining in which chip discharge space is narrow because of the machining form. Therefore, the cutting insert is required to have a structure permitting smooth discharge of generated chips.
The cutting insert described in Patent Document 1 has a rake face, and a clamp face formed on the top surface of an insert body part, a flank face formed on the side surface of the insert body part, and a cutting edge formed at the intersection between the rake face and the flank face. The rake face has a first rake face being continuous with the cutting edge, and a second rake face (an inclined wall) which is located in the back of the first rake face and extends therefrom in an inclined surface shape toward the clamp face. Projections (convex parts) are formed at portions of the first rake face which are adjacent to both ends of the cutting edge, respectively.
In addition to the structure of the above cutting insert, the other cutting insert described in Patent Document 1 further has on the rake face a concave part extending from the portions adjacent to both ends of the cutting edge to the tip end portion of the clamp face so as to divide the second rake face. Projections are formed in the concave part and at the portions adjacent to both ends of the cutting edge, respectively.
These two cutting inserts described in Patent Document 1 have the following problem. That is, in these cutting inserts, the projections are formed at the portions of the first rake face adjacent to both ends of the cutting edge, and hence the opposite ends of a chip are lifted and the widthwise center of the chips is subjected to a large force by which the chip is downwardly deformed (contraction action). Consequently, the chip is deformed into a substantially U-shaped cross section.
However, because the second rake face is constituted by a flat surface, the contact area between the chip deformed by the first rake face and the second rake face is large to thereby increase frictional resistance. Therefore, these cutting inserts have the problem that the chip is liable to accumulate on the rake face, and chip discharging performance is deteriorated, thus causing damage to the insert.
In the other cutting insert of Patent Document 1, by the concave part extending up to the clamp face so as to divide the second rake face, chips are also allowed to be deformed downward at the widthwise center thereof on the second rake face. With this insert, the chip deformed by the projections is also subjected to the contraction action on the second rake face.
The second rake face is formed in the inclined surface shape. Hence, on the second rake face, the chip is subjected to the force by which the chip is curled upward along the chip length direction (curling action), in addition to the contraction action.
That is, in the other cutting insert of Patent Document 1, the generated chip is subjected to the contraction action on the first rake face, and then subjected to both the contraction action and the curling action on the second rake face. Thus, on the second rake face, these two forces substantially orthogonal to each other are exerted onto the chip, and hence the chip discharge direction is unstable. As a result, there has been the problem that the chip is accumulated into the portion of the concave part which is located on the second rake face.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-322010